Integrated Management of Poinsettia Pests:Fungus Gnats

Richard K. Lindquist, The Ohio State University

INTRODUCTION

Fungus gnats and poinsettias, like sauerkraut and pork,seem to be a natural pair. Fungus gnats are among themost common insects found on poinsettias, especiallyduringpropagationand shortly after potting the fall crop.Why the fuss over fungus gnats? There are severalreasons: 1. Fungus gnat larvae can damage healthy roots,stunting or killing young seedlings and cuttings duringpropagation and after potting. 2. Fungusgnat adults andlarvae can carry several pathogens that cause root rotdiseases, so there is potential for increased disease problems when fungus gnats are around. 3. Fungus gnat adults,because they can fly around, are nuisances in homes,offices, and retail shops. Therefore, fungus gnats can affectplant development and value both directlyand indirectly.

SUMMARY OF GENERAL BIOLOGY,DETECTION, AND IDENTIFICATIONFungus gnats have four developmental stages: egg, larva,pupa, and adult. Fungus gnat adults are about 1/8-inchlong with long antennae. There is a distinct "Y" vein oneach wing. Adults often can be seen flying near the pottingmix surface or walking over the surface. Sometimes adultscan be found well away from any potting mix or soil.Females can produce 100 to 200 oval, white eggs that arelaid within protected areas of the growing media surface.Larvae feed on fungi, decaying organic matter, and/or plantroots and stems. Larvae pass through four stages (instars),and they are about .5-inch long when fully grown. Fungusgnat larvae are translucent white with shiny black heads.Late-stage fungus gnat larvae are nearly pure white and canoften be seen on the potting mix surface. After 10 to 14days, the pupal stage is formed, usually just beneath thepotting mix surface. Adults normally appear three or fourdays later. The egg-to-adult cycle is completed in three tofour weeks, depending on moisture and temperatures.Adults can be detected by using yellow sticky traps, placedeither horizontally (sticky side up) on the potting mixsurface or vertically within the plant canopy. Horizontaltraps may be more effective, based on data shown in Figure1. This figure shows far greater numbers of fungus gnatadults on horizontal sticky traps compared to vertical trapsin the plant canopy. If traps are used, they must be checked(and possibly replaced, depending on fungus gnat numbers)at least weekly. It is not necessary to count the trappedfungus gnat adults. The important thing is to note whethernumbers are increasing or decreasing from week to week.

Larvae can be detected by placing potato pieces on orslightly in the potting mix surface. Leave the potato piecesin place for 72 hours to attract larvae. When the potatopieces are lifted, larvae can be seen on them as well as on

Page 12, Southeastern Floriculture

the potting mix surface adjacent to the pieces. Becausefungus gnat larvae are not evenly distributed among allpots or flats, there is a good chance that larvae will not bein the area of the potato slices.

PLANT INJURY POTENTIALAs mentioned earlier, fungus gnat larvae can injure plantsduringand after propagation. Figure2 showsresultsof anexperiment comparing fungus gnat larval sampling (usingpotatowedges - one is almost temptedto call this the"wedgiemethod") and poinsettiahealth. The plant healthindex was a visual assessment of root and foliage appearance, and might be better called a plant quality rating. Thiswas developed by Millie Casey and Terry Moore, researchtechnicians in my laboratory. It should probably be calledMillie and Terry's Plant Health Index.

The data show that higher numbers of fungus gnat larvaeare associated with lower plant health ratings. Plantdisease (Pythium or Rhizoctonia) did not appear to be afactor in these ratings, based on the use of test kits duringthe experiment, as well as application of fungicidedrenches. So, if these data are correct and the plant healthdifferences are due to fungus gnat larval feeding, it showsthe importance of keeping fungus gnat numbers low toimprove plant quality.

INTEGRATED FUNGUS GNAT MANAGEMENT

Three management categories can be used to managefungus gnats: cultural, chemical, and biological. There arepossibilities in all the categories to reduce problems withfungus gnats, but the best results will occur when two ormore approaches are used.

Cultural Management. Greenhouses with wet and/orweedy areas may have more problems with fungus gnats.Keep these areas weed-free, clean up spilled potting mixand tipped pots, and keep problem areas as dry as possible.Some growers apply hydrated lime under benches, using aslurry of 1 to 2 pounds per gallon of water in a sprayer.Limestone F, from W.A. Cleary, is said to be effective aswell. Limestone F is a flowable formulation and is

therefore much easier to apply than dry hydrated lime.Chuck Powell, Plant Health Advisory Services,Worthington, Ohio, suggests using 10 gallons of LimestoneF per acre in enough water to produce a heavy spray. Thisshould also help reduce numbers of shore flies and westernflower thrips.

It is well known that there are differences in the number offungus gnats produced by different potting mixes. These

differences may be due to attractiveness of a mix, variationin fungus gnat survival, or both. Coir-based (coconut fiber)potting mixes have been mentioned as having fewerproblems with fungus gnats. This may be because thesurface of coir mixes dries faster and is less attractive to

egg-laying adults. Figure 3 shows fungus gnat survival indifferent potting mixes, using "choice". The graph bars andnumbers represent fungus gnat larvae in different pottingmixes when adults were allowed to select among mixes.Similar results were obtained when larvae were actuallyplaced in the different mixes, in a non-choice test.

Coir alone (100% coir) did not reduce fungus gnat numbersin either experiment, compared with two other commercialmixes; but a mixture of coir and MetroMix 366, as well as

a mixture of coir and perlite, did reduce fungus gnatnumbers. The reasons for all of this have not been deter

mined, but as they say, we're working on it.

Chemical Control. Some excellent products are registeredfor fungus gnat control in greenhouses. Conventionalinsecticides registered for fungus gnat larval control areKnox Out GH, and DuraGuard. Both are micro-encapsulated formulations, of diazinon and chlorpyrifos, respectively. Nonconventional, or biorational products for larvalcontrol include: Adept (diflubenzuron), Azatin(azadirachtin), Citation (cyromazine), Enstar 11(kinporene), Gnatrol (Bacillus thuringiensis israelensis),and Precision (fenoxycarb).

Results of an experiment using a single application of someconventional and nonconventional products for fungus gnatcontrol are shown in Figure 4. The treatments were applied14 days after plants were removed from mist propagation.The bars in the figure represent the average total number offungus gnat larvae observed on potato wedges over a five-week period after applications. This trial also includedMarathon 1% G, which is not registered for fungus gnatcontrol. It should be emphasized that Marathon is notregistered for this use, and it was applied at approximatelydouble the suggested rate for the pot size used. All potsreceived a Subdue drench in addition to the pesticides. Asthe figure shows, control was good with all of the productsincluded in the experiment, compared to Subdue only anduntreated pots.

For adult control, several products are registered, includingAstro (permethrin), Decathlon (cyfluthrin), resmethrin(Resmethrin EC26, PT 1200), pyrethrum (PT 1600A, PT170), and Taistar (bifenthrin). A combination programinvolving larval and adult control will probably be necessary, if adults are present in significant numbers when youbegin a control program.

Biological Control. There are two options for biologicalcontrol using beneficial organisms: entomopathogenic

nematodes and predatory mites (Hypoaspis). Both arecommercially available for fungus gnat control. Nematodes are tiny, wormlike creatures that usually live in thesoil. Entomopathogenic nematodes are specialized forattacking the larvae of certain insect species. The nematodes do not kill the larvae directly, but enter the bodythrough openings and release bacteria, which multiplyrapidly and kill the insects. Worldwide, several nematodespecies and isolates within species are now available forfungus gnat control. In the United States, Steinernemafeltiae strains are commonly used and are available underseveral trade names, including Entonem, Scanmask, and X-Gnat (although the production of X-Gnat is temporarily onhold).

Predatory mites in the genus Hypoaspis are also used inplace of, or along with, nematodes. They are availablecommercially from several insectaries. These tiny mitesare general predators that live in the soil or potting mix.They can survive without prey, so they can be introducedinto the potting mix before planting. Nematodes and mitescan be used together.

Results of an experiment using nematodes and predatorsfor fungus gnat control over an 11-week period are shownin Figure 5. Nematodes were applied three times at weeklyintervals beginning in the second week after planting. Eachnematode application was one billion nematodes per acre(three billion total). Hypoaspis predators were introduced

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Fungus Gnat Adults Trapped

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Fungus Gnat Larvae and Poinsettia HealthWeekly Averages of Larval Trapping

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Figure 1. Fungus gnat adultstrapped on horizontal andvertical sticky tapes over anine-week period.

Figure 2. Comparing poinsettia plant health (R) withfungus gnat larval numbers(L). *There were 10 insecticide treatments, with and andwithout Subdue drenches,plus two control treatments.

Figure 3. Fungus gnat larvae in different potting mixesafter adults were allowed to

"choose" among them for egglaying.

Adept 1.3 oz

Citation 1.7 oz

Azatin 6.6 oz

PT265100oz

Marathon 3.2g

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Fungus Gnat Control on Poinsettias

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Southeastern Floriculture, Page 15

once, immediately after planting. Predatory mites wereintroduced at the very high rate of 25 per 4.5-inch diameterpot (200/square foot). This is 10 to 20 times the rate nowsuggested by some commercial insectaries for these mites.Obviously, the mites are more effective than we firstthought. Both beneficial organisms reduced fungus gnatadult emergence, compared to emergence from untreatedpots.

To summarize, fungus gnats can be controlled with acombination of cultural, chemical, and biological approaches. It is important to realize that the first few weeksof the crop are when fungus gnats probably will cause themost trouble, and therefore are the most important times forfungus gnat control. If past experience has shown thatfungus gnats are likely to cause problems, use a preventiveapproach. To help monitor the control level, rememberthat yellow sticky traps will indicate fungus gnat adultactivity, and potato pieces placed on the potting mixsurface will help detect larvae. OFA, Bulletin No. 813.

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